The present disclosure relates to the field of polarization spectroscopy. More specifically, it provides a means by which vacuum ultraviolet (VUV) or shorter wavelength of light may be polarized in a highly efficient manner. In one embodiment the techniques disclosed can be used to linearly polarize broad band VUV light in such a means as to afford maximum optical throughput. As used herein VUV light includes, generally, wavelengths of light that are about 190 nm and less wavelengths.
A polarizer is a device that converts a beam of electromagnetic radiation with undefined or mixed polarization into a beam with well-defined polarization. Polarizers are employed in a wide variety of optical instruments covering a diverse range of applications. Polarizers can be divided into two general categories: absorptive, where the unwanted polarization states are absorbed by the device, and beam-splitting, where the un-polarized light is split into two beams with differing polarization states.
Perhaps the simplest absorptive polarizer is the wire grid polarizer, consisting of a fine array of narrow metal lines oriented in a plane perpendicular to the incident beam. Electromagnetic waves which have a component of their electric fields aligned parallel to the wires induce the movement of electrons along the length of the wires. Since the electrons are free to move, the polarizer behaves in a similar manner as the surface of a metal when reflecting light; some energy is absorbed by the wires, resulting in Joule heating, while the rest is reflected backwards along the incident beam. For waves with electric fields perpendicular to the wires, the electrons cannot move very far across the width of each wire; therefore, little energy is absorbed or reflected, and the incident wave is able to travel through the grid. Since electric field components parallel to the wires are absorbed or reflected, the transmitted wave has an electric field purely in the direction perpendicular to the wires, and is thus linearly polarized.
In practice, the separation distance between the wires must be substantially less than the wavelength of the incident radiation, and in turn, the wire width should be a small fraction of this distance. Consequently, wire-grid polarizers are generally only used in conjunction with longer wavelength radiation (i.e. at microwave, far- and mid-infrared wavelengths). To a lesser degree, wire-grid polarizers capable of operating at visible wavelengths can be realized using very tight pitch metallic grids created via advanced lithographic techniques. Unfortunately, further extension of this approach to yet shorter wavelengths in the vacuum ultraviolet (VUV) regime is simply not practical as a result of the very small dimensions required.
Certain types of materials, like some crystals, are known to absorb more light in one incident plane than another. This anisotropy in absorption, called dichroism, can also be employed to control polarization. Generally speaking, this effect is wavelength dependent and by definition results in significant losses if a high degree of polarization is sought. As a result, dichroic crystals are not frequently employed in polarizer applications.
Beam splitting polarizers typically divide un-polarized incident light into two beams of differing polarization through use of reflection or refraction. Reflection-based polarizers exploit the fact that when light reflects at an angle from an interface between two transparent materials, the reflectivity is different for light polarized in the plane of incidence and light polarized perpendicular to it. Hence, simple polarizers can be constructed by tilting a stack of transparent plates at an angle relative to an incident beam. However, to achieve even moderate polarization in this manner it is necessary to incorporate many such plates, or to greatly increase the angle of the plates relative to the incident beam. In either case the intensity of the resultant polarized beam is typically quite low.
Refractive polarizers exploit the birefringent properties of crystals to separate un-polarized incident light into beams with differing polarizations. These devices usually consist of two prisms judiciously cut and arranged such that the ordinary and extraordinary rays are split into orthogonal linear polarization states. The prisms are typically cemented together or separated by a small air gap.
There are a wide variety of such devices, each tailored for use in a specific wavelength region or application. The most prevalent design for operation in the VUV is that of the Rochon polarizer. This device typically consists of two prisms of single crystal magnesium fluoride (MgF2) which are optically contacted such that the ordinary beam passes through the prism unhindered; while the extraordinary beam exits the element at a small angle. These devices are manufactured by a select number of vendors including the Karl Lambrecht Corporation of Chicago, USA and Bernhard Halle Nachfolger GmbH of Germany.
Rochon polarizers capable of operating in the VUV are manufactured almost exclusively from MgF2 as a result of its relatively high transmittance at short wavelengths. In spite of this favorable characteristic the performance of such devices is far from optimum, owing to the material's low birefringence. Unfortunately materials like quartz, which are considerably more birefringent than MgF2, simply do not transmit at these wavelengths. Consequently, MgF2 Rochon polarizers must employ large cut angles in order to produce sufficient separation between the ordinary and extraordinary beams. As a result, polarizers with disproportionately large length to clear aperture ratios are required, significantly limiting optical throughput at short wavelengths.
In view of these shortcomings, there would be great benefit in the development of a highly efficient VUV polarizer. Such a device would be capable of effectively separating un-polarized or randomly-polarized light into beams with orthogonal polarization states such that at least one of the beams may be fully utilized. The techniques disclosed herein would maximize optical throughput by minimizing the optical path length traversed by the light while inside the refractive materials.